Rett Syndrome (RS) is a progressive neurological disorder which develops in females after apparent normal psychomotor development for the first six months of life. The etiology and pathogenesis of RS are unknown. There is no clearly effective treatment. Concerted laboratory and clinical investigations are essential for the elucidation of this problem. The overall goal of this project is to expand the data base with respect to this syndrome, to clarify our understanding of the individual components of the disorder, and, ultimately, to define the etiologic basis. As such, this project both adds to and receives strength from the other components of the program. The specific objectives of this project concern the study of several clinical manifestations of RS in order to define their character and natural history and to document intervention outcome. Abnormalities of cardiac conduction and repolarization will be evaluated from prospective recordings of ECG. Arrhythmias and heart rate variability will be determined from Holter monitoring in Stage II RS patients and age-matched controls. These non-invasive studies will permit determination of the contribution of the autonomic nervous system to control of these cardiac parameters and to sudden death in RS. Video/EEG/polygraphic monitoring will be performed in RS patients with frequent possible seizures. The character and frequency of seizures vs non-seizure events will be determined. These studies will provide new information regarding RS and ultimately may lead to improvement in the quality of life for RS patients. The data will be correlated with data from other projects to better characterize the clinical expression of RS and gain further insight into the underlying pathophysiology of RS.